Method of grain refining zinc



Dec. 26, 1967 L. J. BONlS 3,36 65 METHOD OF GRAIN REFINING ZINC Filed Jan. 25, 1965 FIG. I PEG. 2

FIG. 4

INVENTOR- L ASZ LO J. BONIS AT TOR N EYS United States Patent 3,360,366 0F GRAIN REFINING ZINC Laszlo J. Bonis, Brookline, Mass., assignor to Ilikon Corporation, Natick, Mass., a corporation of Delaware Filed Jan. 25, 1965, Ser. N 0. 427,982 4 Claims. (Cl. 75-178) METHOD ABSTRACT OF THE DISCLOSURE My invention relates to a method of grain refining zinc with yttrium and to a novel class of intermetallic zinc-yttrium compounds.

It is an object of my invention to provide a method by which zinc and rich zinc alloys characterized by a fine grain size may be prepared. It is also an object of my invention to provide novel zinc-yttrium compounds and alloys. Other objects and advantages of my inven tion will be apparent from the following more detailed description taken in conjunction with the accompanying drawing wherein FIG. 1 is a photomicrograph of a sectionalized casting of pure zinc of X magnification; FIG. 2 is a photomicrograph of a sectionalized casting of pure zinc to which has been added 0.05 weight percent of yttrium as a grain refiner of 5 X magnification; and FIGS. 3 and 4 are photomicrographs of portions of the casting shown in FIGS. 1 and 2 respectively at a magnification of 40x.

Zinc and zinc alloys are widely used in industry for castings, for protective coatings on base metals, such as galvanizing and for other purposes. Die castings usually require a high-grade zinc whereas protective coating. may employ a lower or less pure zinc or zinc alloys. Particular zinc alloys are usually selected for special physical or electrical properties, for example, a zinc alloy composed of 99.9% zinc. 0.03% cadmium, 0.02% lead is often used for dry battery casings.

It is a characteristic of pure zinc e.g. over 95% by weight zinc and highly zinc rich alloys e.g. 80% by weight zinc and above, that the zinc and zinc alloy material solidifies in the form of large grains. The particular grain size formed depends in part on the rate of cooling of the zinc or zinc alloys. For example, in castings having a low surface area to volume ratio, for example spheres, very large grains of from 1 to 3 inches or more may be formed. Molten zinc and zinc alloys poured into a thin layer of sheet material and rapidly cooled may have typical zinc grains of as low as to A1" in size. A coarse granular structure in zinc or zinc alloys is undesirable particularly in thin sections where this structure can result in porosity or physical weakness. Thin sections of zinc and zinc alloys having large grain size often tend to exhibit basal plain cracks. For many applications, as for example, dry battery casings it would be of considerable advantage to be able to cast the zinc or zinc alloy structure in a manner which would result in very fine grains. At present the prime method of reducing the grain size of zinc and zinc alloys is to prepare the material in thin sheet form and to cool the material rapidly. It would be highly desirable and most convenient to produce zinc or zinc alloys having a fine grain structure by the use of a simple additive or grain refiner which could be added to the molten zinc during the usual process of casting. Such a procedure Would simplify exotic mold design, eliminate the need for quick or rapid chilling of the molds, increase the thickness of sections which would exhibit the line grain structure, and provide zinc and zinc alloys of increased tensile strength and enhanced flexure strength. A grain refined zinc would therefore in general produce a superior zinc product either in sheet or casting form with little or no additional cost in processing or the special designing of casting or other equipment.

I have found that under similar conditions of crystallization or solidification, zinc and zinc alloys to which have been added a small grain refining amount of yttrium will provide materials having a fine grain size. I have discovered that the addition of yttrium to molten zinc significantly enhances the reduction of zinc grain size usually by at least an order of magnitude and provides a simple inexpensive method of grain refining zinc and rich zinc alloys. For example, I have found that the simple addition of yttrium to the molten zinc prior to casting gives a fine grain size regardless of the particular manner of cooling the zinc as the same method with pure zinc or high grade zinc alloy.

The zinc-yttrium, alloy formed, provides a material of good or enhanced tensile strength and greater flexure strength. A grain refining amount of yttrium may range typically from about 0.001 to about 5.0 percent by weight of yttrium in the zinc, depending upon the nature of the zinc and zinc alloys and type and extent of grain refining desired. For example, yttrium in the amount of about 0.025 to 2.0 weight percent usually provides satisfactory grain refining in zinc of 99 percent by weight and higher. Yttrium is relatively expensive and therefore it is preferred that the yttrium be directly added to and dissolved in the molten zinc or zinc alloy prior to crystallization and preferably by the addition of a yttrium metal or a yttrium master alloy. The yttrium may be added directly to the melt as a fine powder or as a molten liquid.

Due to the reactive and pyrophoric nature of fine yttrium powder the bell immersion technique or other means of inhibiting reaction of the yttrium with the atmosphere should be employed in the addition of the yttrium to the zinc melt. One preferred and simple method of incorporating the desired amount of yttrium is to employ a master alloy comprising zinc and yttrium. The master alloy may contain yttrium up to the solubility limit of yttrium in the zinc. This master alloy or one containing the desired multiple quantity of yttrium may be added to the zinc melt to provide the necessary grain refining. However, it is within the contemplation of my invention that any yttrium compound may be employed which compound decomposes, reacts or otherwise provides free yttrium in the zinc melt. For example, suitable yttrium compounds may include yttrium carbonate, yttrium nitrate, yttrium chloride, yttrium bromide and other yttrium salts and compounds. It is believed that yttrium acts as an oxygen scavenger in the zinc melt and that the subsequent formation of insoluble yttrium oxide acts as a plurality of many fine nucleating centers which centers carry on a competitive crystallization reaction as the zinc melt is cooled, and crystallizes thereby promoting the formation of a large number of fine grains rather than a small number of larger size zinc grains. It is further believed that in pure zinc or zinc rich alloys that as the zinc solidifies there are few competing grains and few nucleating centers and accordingly larger size grains are thereby formed.

The addition of a small amount of yttrium to the zinc melt forms a zinc-yttrium alloy or intermetallic compound or system above the melting point of the alloy. This intermetallic compound results in the grain refinemen-t of zinc. My grain refined zinc yttrium alloys on solidification have a typical grain size about 0.20 or smaller such as from 0.20" to 0.01". My zinc yttrium compounds may be employed in preparing a wide variety of zinc-yttrium castings, sheet material and other forms.

A representative example of how yttrium may be employed to carry out the objects of my invention is as follows:

Example 1.-800 grams of pure 99.995% zinc was melted in a ceramic crucible maintained at about 850 F. in a small electrically heated pot furnace. When the melt had attained the 850 F. temperature, a clean 25 ml. steel combustion crucible was dipped beneath the melt surface, half filled and set aside to cool slowly to room temperature. To the residual 750 grams in the ceramic crucible was added 0.375 grams (0.025 weight percent) of powdered yttrium metal. The melt was stirred with a quartz rod for 15 minutes to insure complete dissolution of the yttrium metal in the zinc melt. Another clean 25 ml. zinc combustion crucible was then dipped into the melt, half filled, set aside to cool slowly to room temperature in a manner similar to the first crucible. The solidified castings after cooling were removed from the crucible, cut into sections, the face of the sections polished by levitated alumina suspended in water and applied to a felt pad polishing wheel, and then after polishing, etched with dilute hydrochloric acid. Photomicrographs of the cut, polished and etched sections at 5 X and 40X magnifications as shown in FIGS. 1 through 4 were then made.

FIG. 1 shows a photomicrograph of a section taken from the 99.995% pure zinc without the addition of the yttrium grain refining additive, while FIGURE 2 shows a photomicrograph of a section cast in identical manner, but which had the addition of 0.05% yttrium metal. As is demonstrated by the photomicrographs the addition of yttrium considerably reduced the grain size of the castings as compared to the grain size of the pure zinc casting made in the same manner. The effect of the addition 4 of yttrium on grain size is even more clearly shown in FIGURES 3 and 4 taken at a greater magnification wherein FIGURE 3 shows the extremely large size grain crystals of the pure zinc while FIGURE 4 illustrates the very fine grain structure of the zinc-yttrium system. Experimental castings with the use of 0.5 and 0.1 and 0.15 weight percent yttrium in zinc produced similar results.

My invention permits the grain refining of zinc and zinc alloys by the simple method of adding small grain refining amounts of the rare earth metal yttrium to the zinc melt. By my invention it is now possible to prepare die castings which castings have said tensile strength and increased fiexure strength, and other desirable properties not hitherto possible or only possible by additional time consuming processing steps.

What I claim is:

1. A method of reducing the grain size of zinc and zinc alloy products, said zinc alloys containing at least 80 weight percent of zinc said products being formed from a melt, which method comprises adding to the melt an amount of yttrium in the range of 0.001 to about 5.0 percent by weight.

2. The method of claim 1 in which said yttrium is i added as a powdered metal.

3. The method of claim 1 in which said yttrium is added in the form of a master alloy of yttrium and zinc.

4. The method of claim 1 in which said yttrium is added in the form of an yttrium salt.

References Cited Transactions of the Metallurgical Society of AIME, volume 227, August 1963, pp. 910916.

Arnes Laboratory Research and Development Report, IS 193, June 1960, pp. 78-88.

Argonne National Laboratory, ANL 6029, June 1959, p. 47.

CHARLES N. LOVELL, Primary Examiner. 

1. A METHOD OF REDUCING THE GRAIN SIZE OF ZINCE AND ZINC ALLOY PRODUCTS, SAID ZINC ALLOYS CONTAINING AT LEAST 80 WEIGHT PERCENT OF ZINC SAID PRODUCTS BEING FORMED FROM A MELT, WHICH METHOD COMPRISES ADDING TO THE MELT AN AMOUNT OF YTTRIUM IN THE RANGE OF 0.001 TO ABOUT 5.0 PERCENT BY WEIGHT. 